Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of gas turbine engines. A gas turbine engine draws in and compresses air with an axial flow compressor, mixes the compressed air with fuel, burns the mixture, and expels the combustion product through an axial flow turbine that powers the compressor. The compressor includes a disk with blades projecting from its periphery. The disk turns rapidly as part of the rotor, and the curved blades draw in and compress air in somewhat the same manner as an electric fan.
Since it takes energy to rotate the gas turbine at high speeds, any efforts to reduce the weight of the gas turbine will improve the efficiency of the gas turbine. More importantly, reducing the weight of rotating components reduces the stresses of the components and enhances the reliability of the gas turbine. One of the areas in which weight can be reduced is the compressor. Compressor components such as compressor airfoils, which include both compressor blades and compressor vanes, are made from steel and iron-base alloy parts that are relatively heavy. Efforts have been made to reduce the weight of these steel and iron-base alloy parts by producing hollow airfoils. However, these airfoils still afford opportunity for weight reduction.
Other attempts for reducing the weight of compressor airfoil components have included both metal matrix composite components (MMCs) and polymer composite blades. Fiber composite blades have been utilized, such as the fan blades described in U.S. Pat. No. 5,375,978, which is modified to include a metallic protection strip such as set forth in U.S. Pat. No. 5,785,498, which also helps provide erosion protection for the fan blade and assists in preventing delamination in the event of impact by a foreign object to minimize foreign object damage (FOD). Both of these patents are assigned to the assignee of the present invention. Such blades are light in weight but are very expensive to manufacture, having high scrap rates. Furthermore, these blades are suitable for use in fan applications where the fan is rotating at a much slower speed than a compressor blade. The compressor blade thus is subject to significantly higher stresses than fan blades.
Compressor blades using MMCs have been manufactured using fabric laid up in the traditional manner and covered with a sheath of titanium or clad with other material. These blades also have proven to be expensive to make and lacking in the strength required for land-based gas turbine operations. Other attempts have included a metallic spar having an outer surface reinforced with a metal matrix composite material, the surface exposed to the atmosphere being metal. While these MMC blades prove to have greater strength, the weight reduction is not as great as with blades having fiber reinforced cores.
What is need is a compressor airfoil that provides weight reduction, yet can have sufficient durability and strength to be used for land-based turbine operation. In addition to being light in weight, the airfoil ideally should also be tunable for resonant frequency control. The airfoil also should be easy and inexpensive to manufacture, with a high yield.